Due to environmental regulations being strengthened because of global warming, there is growing interest in the handling of CO2. Therefore, an industry for storing and transporting CO2 and then burying CO2 in offshore oilfields is being established. Accordingly, demand for steel for tanks used for liquefying and storing CO2 gas is rapidly increasing.
At least 7 bars of pressure are required to liquefy CO2 gas. Since gas tanks for liquefying CO2 gas are designed to withstand temperatures of −60° C. or less, the steel for the gas tanks requires high strength so as to bear high pressure and resist external impacts, and also, the steel requires sufficient toughness, even at a low gas temperature. Specifically, according to classification rules, the steel used for the gas tanks is required to have excellent low temperature toughness, even at a temperature of −75° C. or less.
In addition, when gas tanks are manufactured by welding steel, it is important to remove stress from a welding zone. Therefore, as a method for removing residual stress from welding zones, there are provided a Post Welding Heat Treatment (PWHT) method using a heat treatment and a Mechanical Stress Relief (MSR) method for removing residual stress by spraying high-pressure water onto a welding zone. Among these methods, when stress in a welding zone is removed using the MSR method, a base metal zone may be deformed by the water impact, and thus, the yield ratio of the base metal is limited to 0.8 or less. In greater detail, when a level of yield strength sufficient to create deformation or more is applied to a base metal zone due to spraying high-pressure water for removing stress using the MSR method, the ratio of yield strength to tensile strength is relatively high, thereby generating the deformation; that is, reaching the tensile strength, and thus, it is possible to generate breakages. Therefore, the difference between the yield strength and tensile strength is limited to be great.
In particular, since gas tanks should be enlarged, it is difficult to remove stress therefrom using the PWHT method. Therefore, the MSR method is being used at most shipbuilding companies, and thus, steel for manufacturing gas tanks requires a low yield ratio.
Meanwhile, as methods for improving the strength of steel, which is one of the properties required in steel, there are precipitation hardening, a solid-solution hardening, a martensite hardening, and the like. However, these methods are used for strength to be improved but possess disadvantages in that there is a deterioration of toughness.
However, in the case of grain boundary strengthening, it is possible to obtain high strength, and furthermore, it is possible to prevent the deterioration of toughness due to a decrease in an impact toughness transition temperature.
As an example, Patent Documents 1 and 2 suggest a technique involved in the improvements of strength and toughness by refining crystal grains, specifically, a method for refining crystal grains of ferrite by refining crystal grains of austenite. However, there are problems in that the manufacturing conditions therefor are complicated, and also, the effect on refining ferrite is less effective.
In addition, Patent Documents 3 to 7 relate to the techniques involved in the refinement of ferrite due to the heavy rolling of a non-recrystallization region. Among the documents, Patent Document 3 suggests a method for refining ferrite by performing compression processing of 30% or more of a reduction ratio at the temperature range of an austenite non-recrystallization region and then an accelerated cooling during cooling of the heated low carbon steel after heating the low carbon steel. Patent Document 4 suggests a method of implementing the refinement of ferrite, in which the method includes first heat treating a general carbon steel to be a martensite structure and reheating the general carbon steel at the ferrite stable temperature range to process with 50% or more of a reduction ratio per pass. In addition, Patent Documents 5 and 6 suggest a method for implementing micro ferrite, in which the method includes limiting an austenite crystal grain size to be a fixed size by static recrystallization, and rolling with 30% or more reduction ratio per pass in the austenite non-recrystallization region. Patent Document 7 suggests a method for refining ferrite with the reheated low carbon steel at 75% or more of the total reduction ratio through a single-pass or multi-pass around the Ar3 temperature, and for 1 second as a processing time for a rolling pass.
However, these techniques require large reduction per pass in the rolling process that is the main process for manufacturing steel, and in which the time per pass is limited. Therefore, the techniques possess difficult manufacturing conditions. In order to implement these techniques practically, the installations of extra-large rolling apparatuses and control systems are required, and thus, it is difficult to implement them with the existing apparatuses.
The above techniques are involved in the improvements of strength and toughness by refining crystal grains, and thus, when the refinement of ferrite crystal grains is implemented according to these techniques, tensile strength and yield strength are both improved, and thereby, it is impossible to implement a low yield ratio.
(Patent Document 1) Japanese Patent Laid-Open Publication No. 1997-296253
(Patent Document 2) Japanese Patent Laid-Open Publication No. 1997-316534
(Patent Document 3) Korean Patent Publication No. 1999-0029986
(Patent Document 4) Korean Patent Publication No. 1999-0029987
(Patent Document 6) Korean Patent Publication No. 2004-0059579
(Patent Document 5) Korean Patent Publication No. 2004-0059581
(Patent Document 7) U.S. Pat. No. 4,466,842